In order to increase the power reserve, by increasing the number of turns of a mainspring, secured at the inner end thereof to a core formed either by an arbor, which is generally cylindrical, or by a more solid boss, one solution consists in decreasing the diameter of the barrel arbor and of the associated boss, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must be thought through carefully as there is a risk of breakage if the diameter of the core is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbor or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot shoulder. A step portion joined to this pivot shoulder limits the axial shake of the arbor or of the core relative to a plate or to a bar carrying a jewel or similar element.
In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
The issue is thus to reconcile the smallest possible diameter, to allow the largest possible power reserve, with rigidity in the ratchet drive.
U.S. Pat. No. 804,728A in the name of JOHNSON describes a barrel whose arbor carries a threaded hub, which in turn carries a mainspring. Two components are therefore necessary for the internal holding of the spring. The drum barrel is guided by a jewel which is not in a single piece with the arbor. The rewinding ratchet is located on the other side of the barrel bar. The portion carrying the square driving the ratchet is not formed by a collar, but by the end of the arbor which protrudes beyond the bar, on the opposite side to the drum.
CH Patent Application No 83330A in the name of BEAULIEU WATCH CO describes a barrel with no cover, including a core screwed onto an arbor. The drum is loosely fitted onto the arbor, between a shoulder of the arbor and the core. Here too, two components are required for the internal holding of the spring.
CH Patent Application No 295135 A in the name of BRAC AG describes a barrel whose plate includes a support thread for the ratchet.